52<) 



THE AMERICAN NATURALIST [Vol. XLVII 



From these observationts it may be estimated that the 

 period of gestation averages from the middle of October 

 to the middle of March, a period of five months or 150 

 days, an extended gestation period for so small a mam- 

 mal. The young are fully formed at birth, with eyes open 

 and with a complete though not very hard armor. They 

 are able to walk in a more or less uncertain fashion within 

 a few hours after birth. 



Copulation occurs with the female turned on the back, 

 this position being necessary on account of the armor 

 and the ventral location of the genitalia. 



POLTEMBBTONIC DEVELOPMENT 



Our earliest observations dealing with the development 

 of the Texas armadillo revealed the facts that the four 

 embryos are enclosed in a common chorion and that these 

 monochorial quadruplets are always unisexual. These 

 early observations stimulated an investigation of the 

 embryological and cytological conditions that underlie 

 polyembryony and sex-determination. The published 

 accounts carry the history of development through the 

 period of ovogenesis up to the time of fertilization and 

 from the primitive streak stage to birth. The hiatus 

 between fertilization and the formation of the primitive 

 streak is almost completely filled by two sets of obser- 

 vations, one by Patterson, who has secured late cleavage 

 stages and all of the history up to the primitive streak, 

 and the other by the writer, who has described the early 

 cleavage of parthenogenetically developing ova. The 

 observations of Patterson were reported at a meeting of 

 the central branch of the American Society of Zoologists 

 at Urbana in 1912 ; the paper on parthenogenetic cleavage 

 is now in press and will no doubt appear before the pres- 

 ent contribution. By piecing together the subject matter 

 of these separate investigations the writer is able to offer 

 the following account of the development. 



